Currently, with improvements in rail wear and rolling contact fatigue (RCF) performance, rail welds, both flash butt and thermite types have become a source of required maintenance for continuously welded rail used by modern railroads. A rail weld is a joint formed when welding two parent rails together. The production of the rail weld results in a fusion line, which is a location at which the opposing rails are fused or joined together. The production of the rail weld also results in a weld heat affected zone (HAZ) which is formed on either side of the fusion line and which has a microstructure varying from that of a pearlitic microstructure found in the parent rails. The non-pearlitic microstructure of the weld HAZ consists of degenerate pearlite or a sphereodized microstructure with inherently different mechanical properties from that of the parent rails. An initial evaluation of indicated weld defects from rail taken from in a track on the UP Railroad near North Platte, Nebr. confirmed that a weld HAZ, as shown in FIG. 1, and an inhomogeneity in microstructure and mechanical properties resulting from the presence of the weld HAZ is a factor in the formation of defects in and around a rail weld.
With reference to FIG. 1, a cross section of a single-pass weldment outlining a weld metal, or rail weld, formed between two rails is shown along with weld HAZ which surrounds the rail weld. Because of varying thermal conditions which occur as a function of distance from the rail weld, the weld HAZ can be composed of as many as four distinct regions: 1) a grain-coarsened-HAZ; 2) a grain-refined-HAZ; 3) an intercritical-HAZ, and 4) a subcritical HAZ. Each of these regions within the weld HAZ possesses microstructures and associated mechanical and physical properties that make each region unique.
In the grain-coarsened-HAZ region, peak temperatures reached during its formation range from approximately 2000 to 2700° F. (1090-1480° C.). Another way to describe this temperature range in metallurgical terms, is that it extends from much above an upper critical transformation temperature to below a solidus temperature of the rail. There are two main metallurgical conditions that occur in the grain-coarsened-HAZ region: 1) the microstructure is austenite, for the most part; and 2) since the austenite produced is much above the upper critical transformation temperature, grain growth may and often will occur. The amount of grain growth will depend on the peak temperature and time at that temperature, i.e. the higher the peak temperature and the longer the time at that temperature, the larger the austenite grains will grow.
Two significant metallurgical consequences result in the grain-coarsened-HAZ region: 1) since austenite is produced, the potential for transformation to martensite upon cooling exists, where martensite is not a desirable transformation product due to its lack of ductility, toughness, and susceptibility to cold cracking; and 2) as austenitic grain size grows, the resultant room temperature microstructure will be similarly affected, with low temperature notch (Charpy) toughness being significantly changed, i.e. the larger the grain size, the lower the notch toughness.
The sphereodized microstructure formed in the weld HAZ consists of spheres of cementite in a matrix of ferrite with a hardness up to 150 Brinell points less than in the parent rail, and specifically, in a heat treated premium rail. The sphereodized microstructures which are formed create weakened mechanical properties in the weld HAZ which results from heating and cooling regimes employed when forming the rail weld. It would be desirable to discover a rail weld treatment which can reduce or eliminate the undesirable microstructures formed in the weld HAZ in and around a rail weld.